1. Field of the Invention
The present invention pertains to the tailoring of a barrier/wetting layer to increase the  less than 111 greater than  crystal orientation of an aluminum interconnect deposited over the barrier layer, thereby reducing electromigration within the aluminum interconnect.
2. Brief Description of the Background Art
Plasma sputtered aluminum is widely used to form interconnects in semiconductor devices. Reliability of the interconnect is critical to the reliability of the device. One of the factors which seriously impacts reliability of an aluminum interconnect is electromigration, where aluminum atoms are physically transferred by electron current. The transferred atoms leave behind void spaces which gradually increase in size with time, eventually causing failure of the interconnect.
In order to reduce electromigration, several different approaches have been used in the industry. For example, doping the aluminum with copper can significantly enhance aluminum grain boundaries, reducing electromigration. Passivation of aluminum surfaces (where a layer of material which does not tend to electromigrate is deposited over the aluminum surface) is also helpful. In damascene or dual damascene structures, several manufacturers have use an underlayer (frequently referred to as a barrier layer) to reduce electromigration of aluminum deposited upon the underlayer. Typically the underlayer is a Ti/TiN dual layer, where Ti is titanium and TiN is titanium nitride.
There are relationships between the aluminum crystalline structure and electromigration available in the literature. One well known and publically accepted relationship is a formulation published by Vaidya and Sinha in 1981:
MTTFxe2x88x9d[S/"sgr"2]Log[I(111)/I(200)]3
where MTTF is the mean time to failure, S is the average grain size, "sgr" is the standard deviation grain size distribution, I (111) is the aluminum (111) plane diffraction intensity, and I (200) is the aluminum (200) plane diffraction intensity. (See S. Vaidya, A. K. Sinha, Thin Solid Films, Vol. 75, p. 253 (1981).
From this relationship, it is readily apparent that the higher the texture of (the amount of) aluminum crystalline orientation, the longer the mean time to failure. Based on this principal, other methods have been developed in the industry for determining the texture of aluminum (111) in a deposited aluminum layer.
One method used in the industry to determine texture is X-ray diffraction, where crystalline structure of a given material is measured by the way in which the material diffracts X-rays of a known wavelength. A description of X-ray diffraction as it was used in development of the present invention is discussed in detail subsequently herein.
Titanium wetting layers and titanium nitride barrier layers deposited using xe2x80x9ctraditional sputtering methodsxe2x80x9d have been used in semiconductor device structures underlying aluminum and aluminum alloy layers. However, the crystal orientation of aluminum deposited over the surface of a titanium nitride barrier layer, for example, is typically not highly textured (has a more random crystal orientation) and has poor electromigration resistance. The reliability of interconnecting structures is affected by the electromigration of aluminum having low texturing.
The term xe2x80x9ctraditional sputteringxe2x80x9d or xe2x80x9cstandard sputteringxe2x80x9d refers to a method of forming a film layer on a substrate wherein a target is sputtered and the material sputtered from the target passes between the target and the substrate to form a film layer on the substrate, and no means is provided to ionize a substantial portion of the target material sputtered from the target before it reaches the substrate. One apparatus configured to provide traditional sputtering is disclosed in U.S. Pat. No. 5,320,728, the disclosure of which is incorporated herein by reference. In such a traditional sputtering configuration, the percentage of target material which is ionized is believed to be less than 10%, and more typically less than 1%, of that sputtered from the target.
A xe2x80x9ctraditionally sputteredxe2x80x9d titanium nitride-comprising film or layer is deposited on a substrate by reactive sputtering where a titanium target is sputtered with a plasma created from an inert gas such as argon in combination with nitrogen gas. A portion of the titanium sputtered from the target reacts with nitrogen gas which has been activated by the plasma to produce titanium nitride, and the gas phase mixture contacts the substrate to form a titanium nitride-comprising layer on the substrate. When the proper amount of nitrogen is present in the plasma source gas, the titanium and nitrogen concentrations in the titanium nitride-comprising layer is stoichiometric. Although such a traditionally sputtered titanium nitride-comprising layer can act as a wetting layer for hot aluminum fill of contact vias, good fill of the via generally is not achieved at substrate surface temperature of less than about 500xc2x0 C. Traditional sputtering produces overhang along the edges of the opening to a trench or via to be filled, and a higher temperature is required to flow the aluminum sufficiently to overcome the overhang.
U.S. Pat. No. 5,543,357 to Yamada et al., issued Aug. 6, 1996, describes a process for manufacturing a semiconductor device wherein a titanium film is used as an under film for an aluminum alloy film to prevent the device characteristics of the aluminum alloy film from deteriorating. The titanium film is formed by a sputtering process over a substrate surface containing a via hole. Subsequently an aluminum film is sputtered over the titanium film. Next, the substrate is heated to 450xc2x0 C. to 500xc2x0 C. to melt the aluminum alloy film, thereby filling the via hole therewith. The thickness of the titanium film is set to 10% or less of the thickness of the aluminum alloy film and at most 25 nm. In the case of the aluminum alloy film containing no silicon, the titanium film is set to 5% of less of the thickness of the aluminum alloy film. In another aspect of the process, a titanium nitride film is formed over the substrate surface prior to application of the titanium film.
S. M. Rossnagel and J. Hopwood describe a technique of combining conventional magnetron sputtering with a high density, inductively coupled RF plasma in the region between the sputtering cathode and the substrate in their 1993 article titled xe2x80x9cMetal ion deposition from ionized magnetron sputtering dischargexe2x80x9d, published in the J. Vac. Sci. Technol. B. Vol. 12, No. 1, January/February 1994. This article describes relative ionization level for sputtered aluminum as a function of various process variables. Further, there is a less detailed, general description of the deposition of TiN films as thin liners or diffusion barriers along the sides and bottom of high aspect ratio trenches. TiN films deposited at lower ion energies (0-10 eV) are said to be bronze in color with resistivities in the 200 xcexcxcexa9cm range, while films deposited at higher ion energies (20-50 eV) are said to be bright yellow gold in color with resistivities in the 75 xcexcxcexa9cm range, but are said to be characteristic of highly stressed films. Peeling is said to be common at thicknesses over 700 xc3x85, with depositions on circuit topography features delaminating upon cleaving. Further description of a high density plasma sputtering method is provided by S. M. Rossnagel and J. Hopwood in their paper xe2x80x9cThin, high atomic weight refractory film deposition for diffusion barrier, adhesion layer, and seed layer applicationsxe2x80x9d, J. Vac. Sci. Technol. B, Vol. 14, No. 3 (May/June 1996).
U.S. patent application, Ser. No. 08/824,991 of Ngan et al., filed on Mar. 27, 1997, describes a Ti/TiN/TiNx underlayer which enables a highly (111) oriented aluminum interconnect. In particular, all three layers are deposited using ion metal plasma techniques, where a high density plasma is created between the sputtering cathode and the substrate support electrode, whereby an increased portion of the sputtered emission is in the form of ions at the time it reaches the substrate surface. The Ti first layer ranges in thickness from greater than about 100 xc3x85 to about 500 xc3x85; the TiN second layer ranges in thickness from about 100 xc3x85 to about 800 xc3x85; and the TiNx third layer, having a thickness ranging from about 15 xc3x85 to about 500 xc3x85, is formed at the end of the deposition of the TiN layer by turning off the nitrogen gas feed to the process chamber. This technology pertains to a multi-layered barrier/wetting system where each layer is applied from a high density plasma, with the aluminum fill deposited over a temperature range from about 350xc2x0 C. to about 500xc2x0. This application is assigned to the assignee of the present invention and is hereby incorporated by reference.
The present invention is an improvement over the art cited above in that it provides titanium-comprising wetting layers and wetting/barrier structures which enable the deposition of high (111) content aluminum interconnects, while reducing the complexity of the fabrication process and the cost of the equipment necessary for fabrication.
We have discovered that it is not necessary to have a tri-layer wetting layer/barrier layer structure (Ti/TiN/TiNx) of the kind described in several of the references cited above to obtain a subsequently deposited aluminum interconnect layer having excellent electromigration properties. We have discovered a number of different wetting layer or wetting/barrier layer structures which enable depositing of overlying aluminum interconnect layers having  less than 111 greater than  texturing sufficient to provide a Rocking Curve FWHM angle xcex8 of about 1xc2x0 or less. These barrier layers include: 1) Ti alone, where the Ti is deposited using a high density plasma; 2) Ti/TiN, where both layers are deposited using a high density plasma; and, 3) Ti/TiN, where the Ti layer is deposited using a high density plasma and the TiN layer is deposited using standard sputtering techniques or collimated sputtering techniques. These barrier layers can be formed on a substrate in a single process chamber or in two process chambers, depending on the composition of the barrier layer and the features available in a given deposition chamber.
In addition, we have discovered that when it is desired to use a tri-layer barrier layer (for reasons other than electromigration resistance), a Ti/TiN/TiNx tri-layer having at least the initial Ti layer formed in a high density plasma provides excellent electromigration resistance in an aluminum layer subsequently deposited thereover. This is true even when the aluminum deposition temperature is below about 350xc2x0 C., for interconnect applications.
We further determined that the application of bias to a substrate (to attract ions toward the substrate) during deposition of at least the initial Ti layer is beneficial in improving the  less than 111 greater than  texturing of a subsequently-deposited aluminum layer up to a point, after which the  less than 111 greater than  texturing decreases. This effect is generally observed up to be independent of aluminum deposition temperature for deposition temperatures up to about 350xc2x0 C.